1. Field of the Invention
This invention relates to a misfire-detecting system for internal combustion engines, which detects misfires occurring in the engine.
2. Prior Art
Conventionally, various misfire-detecting systems for internal combustion engines have been proposed. The present assignee has proposed a misfire-detecting system, for example, by Japanese Laid-Open Patent Publication (Kokai) No. 5-321748, which contemplates that a transient irregular combustion can occur depending on operating conditions of the engine or traveling conditions of a vehicle in which the engine is installed, and therefore inhibits the misfire determination when the engine coolant temperature or the intake air temperature is lower than a predetermined value, when the engine is operating in a transient condition in which at least one of the intake pipe pressure, the throttle valve opening, the traveling speed of the vehicle, etc. largely changes, or when the engine is operating in a low load condition such as traveling of the vehicle on a downhill road at a high altitude.
When the vehicle is in stoppage (standing condition), the engine is in a no-load condition in which a smaller amount of intake air is supplied to the engine than during traveling of the vehicle and no inertia force acts upon the driving system of the engine so that the combustion state of the engine assumed during stoppage of the vehicle is more liable to influence the rotation of the crankshaft to cause fluctuations thereof than during traveling of the vehicle. In the conventional proposed systems, however, the same predetermined engine coolant temperature value is employed for determining whether or not misfire detection is to be permitted, between during traveling of the vehicle and during stoppage of the same.
As a result, if the predetermined engine coolant temperature value for determining permission of misfire detection is set to a low value enough to conform to operating conditions of the engine during traveling of the vehicle, there is a high possibility that an erroneous determination can be made as to occurrence of a misfire since the combustion state is unstable if the engine coolant temperature is low when the vehicle is in stoppage.
On the other hand, if the predetermined engine coolant temperature value is set to a high value enough to conform to operating conditions of the engine during stoppage of the vehicle, it results in a too narrow engine coolant temperature range in which the misfire detection can be carried out, because the misfire detection can be carried out at a lower coolant temperature than the above range when the vehicle is traveling.